Background
W. D. Gaster was the royal scientist before Alphys, responsible for creating the CORE. Gaster's Followers relay various explanations as to what happened to Gaster. One iteration says that he "fell into his creation," another says that "his experiments went wrong," and the last one says he "shattered across time and space." Characters such as the River Person and Gaster's Followers hint at his existence. Similar to Sans and Papyrus, he speaks in a typeface different from the other characters.
General Information
Name: W. D. Gaster (The "W. D." likely stands for the "Wing Dings" font)
Origin: Undertale
Sex: Male
Age: Unknown
Classification: The Man Who Speaks in Hands, Mystery Man, Impossibility
Species: Monster (formerly)
State of Being: Nonexistence (The mere act of acknowledging his existence or saying his name causes one to be erased or all of reality to reset. Even for the brief moments he can enter reality merely acknowledging him causes him to be completely erased)
Occupation: Former Royal Scientist
Affiliations: Underground (Formerly)
Enemies: Humans (Formerly)
Status: Other (Gaster doesn't exist to the scale below even beings like Chara or Goner Kid. The mere act of acknowledging his existence or saying his name causes one to be erased or all of reality to reset. Even for the brief moments he can enter reality merely acknowledging him causes him to be completely erased. Though he's a being that's heavily implied to constantly being able to escape the void)
Alignment: Unknown (Gaster has not made himself present enough to show his morals, the only known thing about him is his want to escape from the void and that he was initially the doctor helping monsterkind with the core)
Codex Statistics
Tier: 11-C within the void, Unknown within reality, At least 10-A, possibly Higher with his avatars inserted into reality, at least 2-A upon acknowledging his name/existence
Cardinality: None within the void, Finite within reality. Finite with his avatars inserted into reality (His abilities can reach to Beyond ℵ0 through his name/existence being acknowledged)
Dimensionality: Devoid
Attack Potency: Absolute Zero within the void (Gaster doesn't exist to the scale below even beings like Chara or Goner Kid. The mere act of acknowledging his existence or saying his name causes one to be erased or all of reality to reset. Even for the brief moments he can enter reality merely acknowledging him causes him to be completely erased), Unknown within reality (Has no noticeable way of interacting in his true form with reality and has no feat, he does have an in-game stat of 66666, however it is unknown where this puts him, implied to have some part in creating Deltarune), At least Peak Human level, possibly Higher with his avatars inserted into reality (Should be comparable to the other monsters, who can harm Frisk, who is comparable to Chara who survived the fall from the top of the mountain), at least Multiverse level upon acknowledging his name/existence (Naming yourself Gaster causes the entire game to be reset)
Durability: Absolute Zero within the void, Unknown within reality (Gaster is completely unable to be harmed as he doesn't even exist, thus making it completely unknown what he can withstand, but it should be noted he seemingly still does "exist" even during totality destruction), At least Peak Human level, possibly Higher with his avatars inserted into reality
Striking Strength: Absolute Zero within the void, Unknown within reality, At least Peak Human Class, possibly Higher with his avatars inserted into reality
Lifting Strength: Absolute Zero within the void, Unknown within reality
Travel Speed: Absolute Zero within the void, Unknown within reality
Combat Speed: Absolute Zero within the void, Unknown within reality
Reaction Speed: Absolute Zero within the void, Unknown within reality
Stamina: Limitless
Range: Unknown physically, Multiversal with his existence acknowledged (Naming yourself Gaster causes the entire game to be reset)
Intelligence: At least Supergenius (Created The Core which which turns thermal heat into magical electricity and serves as a power plant for the entire Underground. Stated by one of his followers that it took Asgore a long time to find a replacement due to how smart Gaster was, another follower notes Gaster's brilliance was irreplaceable with this he should then be far more intelligent then Alphys. Hacked into the CORE, even overriding Mettaton's hacking in the process, could figured out a highly complicated math problem in a couple of seconds, correctly guessed the number of flies in a jar with a glance, and perfectly guessed the number of n’s in Mettaton’s name in a situation where the number of n’s was rapidly growing, while it was all being asked in a row. Now that he's shattered across space-time he likely has higher intelligence then even before)
Superhuman Physical Characteristics, Incoporeal Aspects (Monsters physiologies show they are mostly made of magic, noting that humans have a "physical form"[1], along with having their SOUL directly attuned to their body[2]), Magic (All monsters are made of magic, being able to do things like make "bullet-patter cards" with their magic[3]), Personal Domain & BFR (When one encounters a monster they get into a fight, a FIGHT is an encounter in a monster's personal domain allowing them to fight humans, this has various rules such as turn-based combat), Creation (Monsters can make weapons, clothes and even food from magic), Limited Spatial Manipulation (Monsters can at times influence the battle box), Transmutation & Healing (Monsters can turn their attacks into healing projectiles), Absorption (Monsters and humans can absorb the SOUL as long as the person isn’t the same species as them), Curtain Fire, Metaphysics Manipulation (Soul Manipulation & Essence Manipulation; All attacks in Undertale are to the SOUL).
Resistance to Physical Attacks (All monsters bodies are attuned to their SOUL thus only attacks that can affect their SOUL can affect them[4]), Metaphysics Manipulation (Soul Manipulation & Essence Manipulation; Can take hits to the SOUL)Genius Intelligence, Cosmic Awareness (Shattered across all of space-time, can listen in on conversations while being outside of reality), Immortality (Life and Death Immunity; The concept of death no longer applies to him), Reality Separation (Reality Separation; Completely outside of Undertale reality), Paradox Existence (Concept and Non-Concept; Exists and doesn't exist at the same time, no longer bound by any concepts as he's completely outside of everything), Omnipresence (Aspatiotemporal Omnipresence; Gaster is shattered across time and space being everywhere and nowhere trapped to view reality from the outside), Personal Nonexistence (Ayin Physiology; Gaster doesn't exist to the scale below even beings like Chara or Goner Kid. The mere act of acknowledging his existence or saying his name causes one to be erased or all of reality to reset. Even for the brief moments he can enter reality merely acknowledging him causes him to be completely erased), Acausality (Causality Immunity; The concept of causality doesn't apply to him as he is outside of it, he can appear at any point in time he wants and is seemingly unaffected by resets), Erasure & Nothingness Manipulation (Conceptual Erasure & Existence Erasure; Even acknowledging Gaster's existence or merely saying his name will cause you to be erased completely from existence), Spatial Manipulation (Created a new room between two points that were previously right next to each other, and erased it afterward), Technology Manipulation (Passive; Frisk's phone shuts down upon running into a follower)
Other
Standard Tactics: Gaster seemingly sends in his followers or an avatar to briefly interact with reality while he listens in one situations from outside of reality.
Weaknesses: Gaster has a weak influence over reality being unable to come back in it for more then a short period of time, he is only able to briefly insert avatars into it.
Explanations
Reason why the SOUL is considered Essence Manipulation.
Gallery
References
Battle Records